You can use friends to help boost, or join a public match and earn it the "hard" way. The more players in the match, the better.
I recommend using a heavy gunner, since he can lay down a heavy rain of fire while taking a lot of punishment. Park yourself in a heavy traffic area and you should be able to take down five before you're dropped. Focus on the enemy's heavy gunner, soldier, and demo man if they're present since they pose a solid threat offensively.